As a precursor for carbon fiber, polyacrylonitrile fiber yarn is widely known. A carbon fiber can be obtained, for example, through an oxidation step in which a polyacrylonitrile fiber yarn as a precursor fiber for carbon fiber is once wound up in a yarn-making step to form a package, the yarn is then unwound from the package, and the precursor yarn is heated and baked in an air atmosphere at a temperature of 200 to 400° C. to be converted into an oxidized fiber yarn; and a carbonization step in which the oxidized fiber yarn is heated at a temperature of 300 to 3000° C. in an inert atmosphere including nitrogen, argon, helium, or the like to be carbonized. Alternatively, the yarn obtained in the yarn-making step is not wound up but is stored in a can or the like, and the stored yarn is then taken out to produce a carbon fiber in the same process as above. The carbon fiber is usually composed of multifilaments constituted of filaments in which the number of monofilaments is 1000 or more.
The application of carbon fibers is being expanded mainly in aerospace applications as a reinforcing fiber for composite materials, and also in sport or general industrial applications. For further expansion of applications, the provision of an inexpensive and high-quality carbon fiber is a critical challenge, and in the step of manufacturing a precursor fiber for carbon fiber, many improvement techniques related to cost reduction due to more effective production have been disclosed. For example, techniques such as thicker yarns to be processed (yarn thickening), narrower yarn width, and smaller interval between yarns (density increase) are effective means for contributing to increase in production amount in limited facilities.
However, in the case where the yarn thickening or the density increase per unit of yarn is easily proceeded, there has been a possibility that especially in a drawing step, a water washing step, a process oil agent application step, or the like, coalescence between monofilaments occurs; fuzz occurs due to the drawing; yarn breakage, insufficient water washing, adhesion irregularity of the oil agent, or the like is caused, so that in the subsequent baking step, fuzz or yarn breakage also occurs to impair processability, and a problem leading to deterioration of physical properties of the resulting carbon fiber may be caused. Therefore, thickened and highly dense yarns are often subjected to convergence improvement treatment between monofilaments such as being intermingled. However, in the case where the yarn is a carbon-fiber-precursor acrylic yarn, intermingling for yarn thickening impairs spreadability of the yarn, so that, for example, when formed into a prepreg sheet, the baked carbon fiber cannot be uniformly formed into the sheet, leading to deterioration in quality.
Therefore, as a method of combining the carbon-fiber-precursor acrylic yarn without impairing the spreadability of the yarn, for example, Patent Document 1 discloses a yarn combining method which includes squeezing a yarn once between two rollers and then twisting the squeezed yarn with a separately provided roller. Further, Patent Document 2 discloses a combining method of a filament bundle, which includes bringing guides into contact with three or more traveling filaments almost in the perpendicular direction in the first stage, doubling the traveling filaments which have passed through the first stage while bringing them into contact with other two parallelly arranged guides in the second stage, and subsequently twisting the doubled filaments at 45° to 90° by using a further provided guide.